High memory usage in my theme, how to find out why?

I'm experiencing a problem of high memory usage in my custom WordPress Theme, and I'm trying to find out what's causing it.

I'm already using Query Monitor to explore the situation, but I can't see anything wrong there. No queries that require a long time, (almost) no duplicates, no errors, nothing. I just see a high memory usage (53,701 kB, 41.0% of 131,072 kB server limit, 131.1% of 40,960 kB WordPress limit), and hear my MacBook Pro fan become very loud when I load the site. Also, no errors in my error log.

So my question here is: what could I do to find out what's causing this memory usage? Are there ways to know exactly which parts of my code are causing the problem?

Thank you.